Schiit Audio Ragnarok integrated amplifier Measurements

Sidebar 3: Measurements

I performed a full set of measurements on Schiit Audio's Ragnarok using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Looking first at the small-signal measurements taken from the loudspeaker outputs (see later), the amplifier's maximum voltage gain into 8 ohms was 26.17dB with the gain set to "26," 15.1dB with it set to "14," and 3.7dB with it set to "0." These gains were identical for both balanced and single-ended input signals, and the amplifier preserved absolute polarity (ie, was non-inverting) for all inputs and outputs. The unbalanced input impedance was close to 10k ohms at all audio frequencies, twice that value for the balanced inputs.

The gain, measured at the unbalanced headphone jack on the front panel, was 21.4, 10.7, and 0.5dB for the three gain settings; the output impedance from this jack varied from 10.7 ohms at 20Hz to 8.5 ohms at 20kHz. The output impedance from the speaker terminals is specified as a very low 0.03 ohm. However, my estimate, taken at an output level of 400mV, was much higher: 1 ohm at 20Hz, and 1.2 ohms at 1 and 20kHz. As a result, the amplifier's response driving our standard simulated loudspeaker varied by up to ±0.8dB (fig.1, gray trace). This graph also shows that the responses into resistive loads were flat up to 10kHz, with a gentle rolloff at ultrasonic frequencies. As a result, the Ragnarok's reproduction of a 10kHz squarewave into 8 ohms (fig.2) was very good, with no overshoot or ringing visible. Commendably, the amplifier's frequency response was not affected by the volume-control setting and was identical when measured from the headphone jacks.

As HR noted, the Ragnarok runs warm. After the amplifier had been tested for a few hours, its front-panel temperature was 106.3°F (41.3°C), while the inset grille on the top panel measured 116.7°F (47.1°C). The channel separation was excellent, at >90dB below 1kHz and still 70dB at 20kHz. The unweighted, wideband signal/noise ratio, ref. 2.83V into 8 ohms and taken with the input shorted to ground but the volume control set to its maximumthe worst casewas good, at 83dB. Restricting the measurement bandwidth to the audioband improved the ratio slightly, to 87.5dB, while an A-weighting filter improved it further, to 100.3dB. A light "bounce" in the traces of the amplifier's noise on the oscilloscope screen suggested the presence of some very low-frequency noise; this can be seen in fig.3, which shows the low-frequency spectrum of the Ragnarok's output while it drove a 1kHz tone at 1W into 8 ohms.

All of these measurements were performed with continuous test signals at low levels. However, when I tried testing the Ragnarok with higher-level signals, I ran into trouble. I always begin testing an amplifier by preconditioning it: running it at one-third the specified power20Wpc in the case of the Schiitinto 8 ohms for an hour before performing any other tests. During this period I monitor the percentage of total harmonic distortion plus noise, to see if there is any significant change with the amplifier hot. The Ragnarok's THD+N started off at around 0.05%, which is what I expected, but as time went on the distortion level rose, and within five minutes had reached 34%! This was true for both channels, for both balanced and unbalanced inputs, and for all three gain settings.

The increasing distortion was due to the amplifier's output stage becoming starved for bias current, meaning that it moved steadily into class-B operation, with dead periods appearing at the zero-crossing points (fig.4). If I turned the amplifier off and waited a few minutes before turning it back on, it began with respectably low distortion, as before, but again quickly moved into starved-bias class-B as it warmed up. The degree of the reduction in output-stage bias depended on the signal level; fig.5 plots the THD+N percentage against time at three power levels: 10V (cyan and magenta traces), 3V (blue, red), and 300mV (green, gray). You can see that only at the lowest level is there no change in the THD+N. Concerned that our sample had broken in transit to my test lab, I contacted Schiit Audio's Jason Stoddard.

"This is actually expected behavior," Stoddard replied. "Let me explain, now that you think we are the least competent amplifier designers on the planet. What you're seeing is the consequence of the microprocessor-managed bias algorithm we're using. Unlike other amps that use microprocessor-based protection, Ragnarok uses its microprocessor for three things: continuous bias management; overcurrent protection; stepped attenuator and input switching control.

"As far as I know, #1 above is unique . . . the algorithm actually biases the output stage in real timemeasuring about 30 times per second, and adjusting 3 times per second, with variable adjustment rates based on the slope of the readings. We did this to eliminate the DC servo, and its signal bleed back to the input stage. The bottom line of this craziness is that the algorithm is very good at distinguishing music signals from overcurrent conditions (and not de-biasing the output stageconfirmed by many hundreds of hours of observation). However, it is not good at distinguishing continuous tones from runaway biastherefore, it continuously adjusts the bias down in the presence of a standard preconditioning test."

I admit, I was suspicious of Stoddard's explanation: I was taught that designers must actively avoid modulating an amplifier's operating parameters with the signal being amplified. Unless they use a very long time constantas with a DC servo, or the sliding bias featured by mid-1990s amplifiers from Krell and Mark Levinsonthe parameters will be correlated with the envelope of the music signal, which can result in distortion and noise modulation. And if the time constant is short enough to react within a few seconds (as in fig.5), I wondered if there would be a history effect; ie, if the amplifier's operating condition would reflect the properties of the music signal integrated over the time constant. I waited until the continuous 1kHz signal used to generate fig.4 had fully starved the output stage of bias current, then replaced it with a 1kHz toneburst signal that alternated between 1000 cycles at 100mV and 9000 cycles at 10mV. As I watched the waveform on the oscilloscope, it took six minutes for the bias to return to its appropriate level for the 10mV signal, with appropriately low distortion.

According to Jason Stoddard, "It takes more than several seconds to de-bias the outputit doesn't adjust that fast. We've tested it extensively with actual music, monitoring the output of the bias control circuit in real time, with no observed significant de-biasing behavior."

This behavior was going to make testing the Ragnarok's production of distortion, which is usually performed with continuous tones, problematic. For example, I tried looking at the Ragnarok's production of harmonic and intermodulation distortion at different continuous levels. Fig.6 shows a spectral analysis of the amplifier's output with a 50Hz tone at 1V output into 8 ohms; fig.7 shows the analysis at 10V. The dominant harmonic at 1V is the subjectively innocuous second harmonic at a low level, 0.05%; but at the higher level, the third harmonic has risen to 0.5%. Similarly, fig.8 shows the spectrum of the Raganarok's output as it amplified an equal mix of 19 and 20kHz tones with a peak amplitude of 3V into 8 ohms. The 1kHz difference product lies at 63dB (0.07%), which is respectably lowbut when I increased the level to 10V peak, a slew of higher-order intermodulation products appeared. Given the action of the microprocessor in adjusting the output-stage bias current, I'm sure that these graphs don't reflect the amplifier's behavior with music signals, which have a much higher crest factor than test tones.

Similarly, when I tried examining how the percentage of THD+N changed with power with both channels driven, the resulting graph was clearly being affected by the action of the bias-adjustment algorithm. For example, fig.9 shows the behavior into 8 ohms: the steady rise in distortion between 100mW and 10W is actually due to the decreasing output-stage bias current. The amplifier does appear to clip above 40Wpc, but I don't feel that is a valid measurement. The picture was similar into 4 ohms, with the sharp rise in THD+N apparent above 65Wpc (fig.10).

I needed, therefore, to use a test signal that more closely resembles a musical waveform yet is still diagnostic. After some experimentation, I decided to use a 1kHz toneburst with 2000 cycles on and 18,000 cycles off (ie, 2s on and 18s off), which would keep the Ragnarok from reducing its output-stage bias. During each dead period, I would manually adjust the level of the input signal to the next higher. Rather than directly measure the THD+N percentage2s is not really long enough for the AP's analyzer to settle and for me to note the readingI would use FFT analysis of the amplifier's output signal to look at the levels of the second, third, and fifth harmonics at each power level.

The result of this test is shown in fig.11, which plots the level of the third harmonic against power into 8 ohms. At low levels, the third harmonic lay at or below 0.03% (70dB), which is respectable behavior. (At 300mW, the second harmonic lay at 0.007%, 83dB, and the fifth harmonic at 0.003%, 90dB, both almost buried in the noise floor.) As the output power increased, the third harmonic slowly rose in a linear manner, but remained low until just above 30W, when it began to rise. (The levels of the second and fifth harmonics didn't change significantly between 300mW and 30W, other than being increasingly separated from the background noise.) The third and fifth harmonics each reached 1% at 48Wpc (16.8dBW) in fig.9, due to the clipping of the tops and bottoms of the waveform. So with this toneburst signal, the Ragnarok doesn't reach its specified maximum power of 60Wpc into 8 ohms (17.8dBW). Whether the shortfall of 1dB is or is not significant will depend very much on the music being playedas will, of course, the behavior of the amplifier's output stage.

Herb Reichert's review reveals that he really liked the sound of the Schiit Ragnarok, so it's probable that the toneburst testing more accurately characterizes the amplifier's behavior than continuous tones. But the correlation between the output-stage bias and the properties of the signal being amplified still raises my eyebrows. The more closely music resembles a continuous tone, the more nonlinear will be the amplifier's performance.

But I'll give the final word to Jason Stoddard: "I explained what we were doing to another prominent amp designer, and he went sheet white and said, 'Um, you are completely insane. How do you differentiate bias from music?' Well, the answer was, 'Many late nights watching the microprocessor dump real-time numbers on a screen, and many iterations of code that help distinguish bias and faults from transient current draws caused by musical output.'"

Well there it is. John Atkinson

Postscript: After the review had been laid out for print, I thought of a way of unmasking the action of the microprocessor that adjusts the output-stage bias current: Play music with high-level sustained tones for five minutes or so, then immediately examine the distortion with a sinewave tone before the microprocessor can react.

I played my recording of Jonas Nordwall playing the Toccata of Widor's Organ Symphony 5 (which has sustained organ-pedal notes) at a level that reached 20Wpc at the climax but for much of the time averaged less than 1Wpc. As soon as the musical climax finished, I immediately switched to a 1kHz sinewave at approximately the same level (14Wpc), keeping one eye on the Audio Precision's THD+N reading and the other on the oscilloscope screen. The measured THD was around 0.7%, but of course it immediately began to rise.

I then played Daft Punk's "Get Lucky," which lasts six minutesapproximately as long as the organ recordingat a level that ranged between 3 and 16Wpc. When I switched to the sinewave, the THD this time (before it began to rise) was 0.5%.

These tests suggest that the microprocessor algorithm has been carefully arranged so that while the bias current will start to decrease with high-level music with sustained tones, the increase in THD won't be audible due to masking, and will decrease once the climax has passed. With less-demanding music that has a much higher crest factor than organ recordings or EDM, it looks as if the Ragnarok's output-stage bias remains high enough not to add distortion, and therefore will not be a significant factor in the amplifier's sound quality. As Mr. Stoddard said.John Atkinson

"...if your expensive stereo is playing some New Orleans R&B—like Frankie Ford's "Sea Cruise," from The Best of New Orleans Rhythm & Blues, Volume Two (CD, Rhino R2 75766)—and you're sitting in your soft chair all serious and thoughtful—well, it's a crap system."

As we are known & prone to exclaim here in N'awlins,Herb:"Yeah, you right!"

I recall that Steve Guttenberg reviewed this amp a number of months ago over at cnet. He gave it a very favorable review also, but why do you guys think the lack of a remote is such a positive? I'm old enough to remember not having a remote for the tv and that was no fun. I have a Peachtree nova 125se, and I tell you I'd like it a lot less if it didn't have a remote.

I agree that the lack of a remote is a deal-breaker for the Ragnarok as a speaker amp. For headphone use a remote isn't needed. From what I've heard Schiit is working on other to-be-announced 2-channel components (with remote controls!) so we'll have to wait and see what gets released later this year.

I need to be able to mute the stereo if the phone rings, if someone knocks on the door, or if my my spouse wants to talk to me without having walk across the room. There are other integrated amplifiers in that price point that have remotes. The Rouge Sphinx 2 remote now has a mute button which now puts it into the running for me. Lack of a remote is a reason for consumers not to buy a product.

This is a company wanting to sell a product and as it has been illustrated here the lack of remote is reason enough for many people not to spend money on this product. Own goal. Shot their own foot. So there....

Shitt does indeed make very nice products, but are hardly the holy grail that you make them out to be, another review in an English magazine liked this amp very much but was hardly blown away by it.

Yes the Rangnarok is an excellent headphone amp, and it is a good amp but there are plenty of others excellent products that are in this same league.

The Rouge Sphinx you compared is a lovely little amplifier, but the Nuprime IDA 8 sounds far more transparent, adds APT X Blue tooth streaming, and a full function remote and costs $295.00 less!

What I find ironic about your review is that you proclaimed this product to be a miracle without comparing it a line of products that are also in many peoples minds, when they talk about sound quality for the money.

I would point to Herb that for not that much more money you can find a great competitor that has a world class Dac built in, is a fantastic headphone amplifier also, and has two matching amplifier pairings plus a full function remote control, if you want to hear what a remarkably transparent, dymanic, package that will shock you with its bass control as well as midrange, you should hear the LS 50 on the Nuprime gear.

We have the LS 50 set up with the DAC 10H headphone amp/dac, $1695 plus the Nuprime STA 9 power amp $649 for a package of $2,344.00 and this combination is remarkably musical, with the Nuprimes very high damping factor you would be amazed at how deep and tuneful the LS 50 sound in the bass with this amplifier.

I always get amused at some of the comments I read about someone not liking something that a reviewer loves and says its not all that? I realize that hearing is in the ear of the beholder and all that, but its quite obvious after the first paragraph that this guy is a Nuprime fan? Because he owns it and he made the decision to buy it, BOY, TRY THIS AMP, IT IS WAY BETTER THAN THAT ONE, I DONT UNDERSTAND WHAT THIS GUY IS TALKING ABOUT? I have heard Nuprime, I bought an IDA 8 when they first came out, so did a bunch of friends that I have. Nuprime cant hold a candle to the Schiit products, the IDA 8 to be specific, is a piece of crap. Nuprime having a world class dac? What Dac would that be exactly? I have owned better integrated amps also, FOR WAY MORE MONEY!! Not even a close comparison, I have heard both, and so have all my audiophile friends. There is simply NO comparison with the Ragnarok and the nuprime crap. Take it for what its worth. Nothing, Just my opinion.

Kudos to John Atkinson for the measurements on this amplifier! The Ragnarok proved ricky to measure, and I know of no other publication that would go to such great lengths to get meaningful measurements. Nice work!

Herb,
Seeing you also did the review of Vinnie Rossi's LIO and also appeared to loved that piece, how would you compare them. While I know it's not totally an apples to apples comparison, your thoughts on these two manufacturer-direct products would be great.

You can't measure our amplifier the same way you can measure every other amplifier on the planet because, eh, yeah, we got this algorithm see, and it does funny things when it knows it's not music playing.

Effectively, what they are telling us is this amplifier cannot ever be properly tested and measured the way all others are and to trust them that the reason the measurements are so bad is not because it's a poorly designed and poorly working amplifier but because it's got this algorithm.

Seriously, they were surely aware of this before sending it out to you to be tested. The way it seems to have happened is you got some very bad results, contacted Schitt who then came away with this story.

I mean, how did they design and test this amp before manufacturing it? How did they know it was the algorithm making a mess of the tests and not bad design? How did they know it was achieving certain parameters during the design and build stage if you can't test the thing?

Yah i really love on the way how your describing this amp specially used to have the Gungnir MB DAC which was made by the same company. Of course not being biased to Schiit but they do really make good quality products considering value to money, anywaze how is this amp compared in sound vs the parasound integrated, recently im planning to invest on the maggie .7. In your opinion which amp does match well w/ the maggies considering my source as vinyl specifically the vpi scout jr. with ortofon red as cart, I'm more lean towards that rich midtone character, i have a rogue audio pharaoh but im planning to sell due to its recessed mid tone character, maybe because of its class d architecture, so hopefully you can provide insights or info to this amp, which of course again I am considering the .7 and a rich tone mid character sound as my preference, thanks a lot Herb more power to you and strereophile